1. Field of the Invention
The invention is related to the field of analog filters, and in particular, to the control of an analog filter operating on a communication channel based on the varying data rate of the communication channel.
2. Statement of the Problem
In a tape drive, a magnetic head reads a magnetic tape and generates a corresponding analog signal that represents the data stored on the tape. The data rate of the analog signal depends on the speed of the tape drive. The speed of the tape drive varies to accommodate system requirements and may also vary because of mechanical imperfections and environmental changes. This speed variance causes a corresponding change in the data rate of the analog signal.
An analog filter processes the analog signal based on a transfer function. For example, a low-pass analog filter may remove noise by attenuating signal energy that is outside of a passband. The passband is centered around the expected data rate, so the analog filter will pass energy at frequencies near the expected data rate, but will remove noise by attenuating energy at frequencies that are not close to the expected data rate.
Unfortunately, as the speed of the tape drive varies, the frequency of the data rate shifts. In the above example, the actual data rate may shift away from the expected data rate, and thus, the frequency for the actual data rate may shift outside of the passband for the low-pass filter. Thus, signal energy at the actual data frequency would be attenuated by the analog filter before the signal is processed by downstream components. For example, the filtered signal may be sampled by an analog-to-digital converter, and the undesirable attenuation may have a significant negative impact on sampling quality. Poor sampling quality adversely affects the overall quality of the tape drive.
In some applications, a current signal controls the frequency of an oscillator in a phase lock loop, and hence, controls the sampling frequency of the analog-to-digital converter in response to the varying data rate. The current signal can also be used to adjust the analog filter settings. Unfortunately, this solution may not be accurate enough for all applications, especially in the presence of changing environmental conditions. For example, this technique may not work well at operational temperatures that are hotter than expected.